Oxygen displacements and search for magnetic order in Sr3Ru2O7

The crystal structure of the layered Sr3Ru2O7 system has been analyzed by neutron powder diffraction methods. The structure is formed by stacking two blocks of distorted SrRuO3 perovskite along the c axis, interleaved with SrO layers. The neighboring corner-sharing octahedra in each double perovskite block are rotated with respect to each other about the vertical axis so that the Ru-O-Ru angle in the RuO2 planes is about 165 degrees rather than 180 degrees. These rotations are correlated within each double perovskite block, but they are not correlated along the c axis, resulting in an intrinsic disorder that emphasizes the layered nature of this material. The resulting structure has the symmetry of space group Pban and lattice parameters a = b = 5.5016(1) Angstrom and c = 20.7194(5) Angstrom at 295 K. Data taken to 9 K showed no evidence of any structural phase transitions occurring below room temperature. We also searched for the development of long-range magnetic order to temperatures as low as 1.6 K, but no evidence of either ferromagnetic or antiferromagnetic long-range order was observed, with an upper limit of 0.05 mu(B) for any possible ordered moment. This result contrasts with a reported ferromagnetic ordering at 104 K with an ordered Ru moment of 1.3 mu(B), which we believe was due to a phase other than Sr3Ru2O7. We also searched for an induced moment, for applied fields up to 7 T, but did not observe any induced ferromagnetic moment within the same experimental limit.